Siloxane resins with oxetane functionality

ABSTRACT

A linear or cyclic siloxane compound contains pendant carbon to carbon double bonds, Si—H bonds, and pendant oxetane functionality. These compounds have two curing temperatures, one associated with a hydrosilation reaction between the carbon to carbon double bond and the Si—H entity, and the other associated with the ring opening of the oxetane.

FIELD OF THE INVENTION

This invention relates to siloxane resins having oxetane functionalityand to these resins in B-stageable compositions for attachingsemiconductor chips to substrates.

BACKGROUND OF THE INVENTION

Included within the processing steps for the fabrication of asemiconductor package, a semiconductor die or chip is mechanicallybonded with an adhesive to a substrate. The fabrication can take placein a continuous series of steps, or the substrate can be prepared withthe adhesive for the mechanical attach, and then held until a latertime.

If the fabrication process is to be interrupted after the deposition ofthe adhesive to the substrate and the assembly held to a later time, theadhesive must be in a solidified form for successful storage. Thesolidified form provides the further advantages of minimal or nobleeding, and better control of bondline thickness and bondline tilt,the bondline being the interface between the chip and the adhesive.

For some semiconductor packaging applications, paste adhesives arepreferred over solid (film) adhesives for process reasons, yet thebond-line and fillet control of solids are desired. In such a case, anadhesive known as a B-stageable adhesive may be used. If the startingadhesive material is a solid, the solid is dispersed or dissolved in asolvent to form a paste and the paste applied to the substrate. Theadhesive is then heated to evaporate the solvent, leaving a solid, butuncured, adhesive on the substrate. If the starting adhesive material isa liquid or paste, the adhesive is dispensed onto the substrate andheated to partially cure the adhesive to a solid state. The applicationof heat at this stage in fabrication is termed B-staging, and theadhesive, B-stageable.

Although there are the advantages to solid adhesives mentioned above,there are also disadvantages. After B-staging and during storage, solidadhesives are prone to absorbing moisture from the air under ambientconditions, or from substrates, especially organic substrates such as BTresins, printed circuit boards or polyimide flexible substrates. Theadhesives also may contain a level of residual solvent or othervolatiles.

At elevated attach temperatures, the absorbed moisture and residualvolatile materials will evaporate rapidly. If this evaporation occursfaster than the vapors can diffuse out of the adhesive, voids or bubblesappear in the adhesive and can be a source of ultimate failure of theadhesive. This creates a need for curable compositions that areB-stageable but that do not promote voiding.

Cyclic or linear siloxane resins, such as those disclosed in U.S. Pat.Nos. 4,751,337, 4,877,820, 4,900,779, 4,902,731, 5,013,809, 5,025,048,5,077,134, 5,118,735, 5,124,423, 5,124,375, 5,147,945, 5,171,817,5,196,498, 5,242,979, 5,260,377, 5,334,688, 5,340,644, 5,373,077,5,391,678, 5,408,026, 5,412,055, 5,451,637, 5,491,249, 5,523,374,5,512,376, have pendant carbon to carbon double bonds introduced throughthe addition of dicyclopentadiene to the linear or cyclic siloxanebackbone. These siloxane compounds have superior stability and very lowmoisture absorption, however, they have less than optimum adhesiveproperties.

Oxetane resins are highly reactive cyclic ethers that can undergo bothcationic and anionic ring opening homopolymerization, and in general,exhibit good adhesion, shrink minimally upon cure, and polymerizereadily. A combination of the properties found in oxetane compounds andin siloxane compounds, in which the siloxane compounds contain Si—Hbonds and carbon to carbon double bond functionality, would be anadvantage in uses requiring dual cure materials, such as in B-stageableadhesives for use in electrical, electronic, photonic, andoptoelectronic applications.

SUMMARY OF THE INVENTION

This invention is a linear or cyclic siloxane compound that contains atleast one Si—H bond, at least one pendant carbon to carbon double bond,and at least one pendant oxetane functionality. When homopolymerized,these compounds have two curing temperatures, one associated with ahydrosilation reaction between the carbon to carbon double bond and theSi—H entity, and the other associated with the ring opening of theoxetane. The first cure occurs at about 110° C., and the second cureoccurs at about 150° C. These curing temperatures are sufficientlyseparated to allow the oxetane functionality to react (ring open) duringa B-staging process and the carbon to carbon double bond and Si—Hhydorsilation reaction to occur at a later stage. These compounds areuseful in compositions for use within the semiconductor packagingindustry, for example, as lid sealants, underfills, films, die attachmaterials, and B-stageable adhesives.

DETAILED DESCRIPTION OF THE INVENTION

The siloxane starting compounds and methods for their preparation aredisclosed in the US patents listed in the Background section of thisspecification. In general, the starting siloxane compounds will have thelinear structure

or the cyclic structure

in which x, y, and z represent the mole percentage of Si—H sites to atotal of 100 mole %. The x sites represent those that remain asunsubstituted Si—H sites. The y sites represent those that will containpendant carbon to carbon unsaturation after hydrosilation withdicyclopentadiene. The z sites represent those that will contain pendantoxetane functionality after hydrosilation with the carbon to carbondouble bond on a compound containing both double bond and oxetanefunctionality. The q sites represent those that will not undergohydrosilation (there being no Si—H bonds on these sites) and is reportedas an integer.

To form the inventive compounds, the starting materials are reacted viahydrosilation with (i) dicyclopentadiene and with (ii) a compoundcontaining oxetane functionality and carbon to carbon double bondfunctionality (the oxetane starting compound). A suitable oxetanestarting compound is allyl oxetane. Reaction of the dicyclopentadienewill occur between the norbornene double bond on dicyclopentadiene and aSi—H site on the starting siloxane compound to add pendant carbon tocarbon unsaturation to the starting siloxane compound. Reaction of thestarting oxetane compound will occur between the carbon to carbon doublebond on the oxetane starting compound and a Si—H site on the startingsiloxane compound to add pendant oxetane functionality to the startingsiloxane compound. These reactions can be run simultaneously orsequentially at the preference of the practitioner and are nonselectiveto the Si—H sites. The resultant compounds are colorless liquids withviscosities less than 1,000 mPa·s at 25° C.

The mol percent for each x, y, and z site will range from 5 to 90% foreach site, with the total being 100% for x+y+z. In general, it ispreferable to have as near equivalent molar levels of substitution ofthe x, y, and z sites as possible to obtain equivalent levels ofcrosslinking in subsequent curing reactions. The unsubstituted sites,represented by q, will range from 1 to 50. As will be understood, inthis specification, with respect to x, y, and z, a site refers to thebond between a hydrogen atom and a silicon atom on the siloxane startingcompound; with respect to q, a site refers to the bond between an alkylgroup (typically methyl) and a silicon atom on the siloxane startingcompound. Also as will be understood, it is not always possible torecite with 100% accuracy the level of substitution at the sites; inparticular, the value for q is approximate, represented by the symbol“˜”.

Examples of compounds that fall within the above description have thecyclic structure

or the linear structure

in which x, y, and z represent the mol % substitution to a total of100%, and q is an integer from 1 to 50.

These compounds may also be blended with other materials having carbonto carbon unsaturation, or with materials that undergo cationic ringopening, such as epoxy, oxetane (other than the inventive compoundsdisclosed herein), benzoxazines, and oxazolines. As will be understoodby those skilled in the art, the carbon to carbon double bonds in theadditional materials will react with the pendant carbon to carbon doublebonds on the inventive compounds, and the materials that undergocationic ring opening will react with the oxetane functionality. Byjudicious choice of additional reactants, one skilled in the art will beable to prepare compounds with curing capabilities to meet many variedend uses.

In the following examples, the linear and cyclic siloxanes werepurchased from Gelest and are indicated by product codes HMS and MHCS.

EXAMPLE 1 Compound 1

The initial charge, a linear siloxane HMS-501 (25.15 g, Si—H=0.162 mol)and toluene (30 mL), was added to a 500-mL 4-neck round bottom flask.The reaction vessel was placed under a N₂ blanket and equipped with anoverhead stirrer and condenser. Stirring was continued until the mixturebecame homogeneous. The temperature was kept between 65° and 70° C.Allyl oxetane (8.424 g, 0.0541 mol), dicyclopentadiene (7.14 g, 0.0541mol), 50 ppm dichloro (1,5 cyclooctadiene) Platinum(II) was charged tothe flask dropwise over a period of one hour. The reaction was monitoredby FT-IR analysis for the consumption of norbornyl double bond (peak at1342 cm−1). The reaction was completed after 17 hours. After thisinterval, the solvent was removed in vacuo (60° C., 0.3 mm Hg) to afforda pale yellow liquid.

EXAMPLE 2 Compound 2

The initial charge, a cyclic siloxane MHCS (25.0 g, Si—H=0.411 mol) andtoluene (30 mL), was added to a 500-mL 4-neck round bottom flask. Thereaction vessel was placed under a N₂ blanket and equipped with anoverhead stirrer and condenser. Stirring was continued until the mixturebecame homogeneous. The temperature was kept between 65° and 70° C.Allyl oxetane (21.385 g, 0.137 mol), dicyclopentadiene (18.084 g, 0.137mol), and 22.5 ppm dichloro(1,5 cyclo-octadiene) Platinum(II) wascharged to the flask dropwise over a period of one hour. The reactionwas monitored by FT-IR analysis for the consumption of norbornyl doublebond (peak at 1342 cm−1). The reaction was completed after eight hours.After this interval, the solvent was removed in vacuo (60° C., 0.3 mmHg) to afford a pale yellow liquid.

EXAMPLE 3 Compound 3

The initial charge, a cyclic siloxane MHCS (25.0 g, Si—H=0.411 mol) andtoluene (30 mL), was added to a 500-mL 4-neck round bottom flask. Thereaction vessel was placed under a N₂ blanket and equipped with anoverhead stirrer and condenser. Stirring was continued until the mixturebecame homogeneous. The temperature was kept between 65° and 70° C.Allyl oxetane (9.622 g, 0.062 mol), dicyclopentadiene (24.54 g, 0.175mol), and 17.5 ppm dichloro(1,5 cyclo-octadiene) Platinum(II) wascharged to the flask dropwise over a period of one hour. The reactionwas monitored by FT-IR analysis for the consumption of norbornyl doublebond (peak at 1342 cm⁻¹). The reaction was completed after five hours.After this interval, the solvent was removed in vacuo (60° C., 0.3 mmHg) to afford a pale yellow liquid.

EXAMPLE 4 Compound 4

The initial charge, a linear siloxane HMS-501 (25.15 g, Si—H=0.162 mol)and toluene (30 mL), was added to a 500-mL 4-neck round bottom flask.The reaction vessel was placed under a N₂ blanket and equipped with anoverhead stirrer and condenser. Stirring was continued until the mixturebecame homogeneous. The temperature was kept between 65° and 70° C.Allyl oxetane (25.27 g, 0.162 mol) and 100 ppm platinumdivinyltetramethyldisiloxane complex in xylene was charged to the flaskdropwise over a period of one hour. The reaction was monitored by FT-IRanalysis for the consumption of Si—H bond (peak at 2160 cm−1). Thereaction was completed after five hours. After this interval, thesolvent was removed in vacuo (60° C., 0.3 mm Hg) to afford a pale yellowliquid.

EXAMPLE 5

Thermogravimetric Analysis (TGA). TGA was used to determine thevolatility of the resins in Examples 1 through 4. A sample of each resinwas heated from room temperature to 350° C. at a ramp rate of 10°C./min. For electronic applications, the weight loss percentage of neatresin preferably is less than 10% at 200° C. For the resins in Examples1, 3, and 4, the weight losses at 200° C. were less than 10 wt %, whilethat of Example 2 was 13% at 200° C. The resin from Example 2, however,is sufficiently reactive to cure into a formulation beforevolatilization occurs.

EXAMPLE 6

Formulations. The compounds from Examples 1, 2, 3, and 4 were eachformulated into a series of three compositions comprising the compoundand initiators. The first composition comprises the Example compound and1 wt % of Rhodorsil 2074. Rhodorsil 2074 is a cationic photoinitiatorcatalyst (available from Rhodia) added to catalyze thehomopolymerization of the oxetane substituent. The second compositioncomprises the Example compound, 1 wt % of Rhodorsil 2074, and 1 wt %radical initiator, USP90MD (available from Akzo Nobel). The thirdcomposition comprises the Example compound, 1 wt % Rhodorsil 2074, and 1wt % benzopinacole, a radical initiator.

The radical initiator behaves as a reducing agent for Rhodorsil 2074during decomposition. As a result, a strong acid is generated whichsubsequently initiates oxetane ring-opening.

The compositions for each Example compound were analyzed separately byDSC to measure kinetic and thermodynamic properties. The results arereported in Table 1 and indicate there is extensive hydrosilation andcationic oxetane ring-opening in the compounds from Examples 1, 2, and 3using these initiator systems. The cationic oxetane ring-opening occursat the lower temperature. There is no hydrosilation of the compound fromExample 4 since 100 mol % of the silicon hydride groups were modifiedwith oxetane functionality; consequently, there is only one cure peakfor these formulations. The data show that these compounds undergo dualcures, with sufficient differential between the two curing temperaturesso that a B-stageable composition is obtainable. TABLE 1 Kinetic andThermodynamic Properties of Compositions Containing Compounds 1, 2, 3,and 4 Cure Temp ΔH Cure Temp ΔH Compositions (° C.) (J/g) (° C.) (J/g)Example 1 Compound 111 −135 236 −64 1 wt % Rhodorsil 2074 Example 1Compound 108 −153 162 −191 1 wt % Rhodorsil 2074 1 wt % USP90MD Example1 Compound 107 −251 137 −116 1 wt % Rhodorsil 2074 1 wt % BenzopinacoleExample 2 Compound 88 −117 161 −353 1 wt % Rhodorsil 2074 Example 2Compound 81 −137 157 −365 1 wt % Rhodorsil 2074 1 wt % USP90MD Example 2Compound 84 −109 164 −455 1 wt % Rhodorsil 2074 1 wt % BenzopinacoleExample 3 Compound 102 −71 167 −161 1 wt % Rhodorsil 2074 Example 3Compound 101 −63 156 −319 1 wt % Rhodorsil 2074 1 wt % USP90MD Example 3Compound 104 −67 158 −322 1 wt % Rhodorsil 2074 1 wt % BenzopinacoleExample 4 Compound 88 −221 218 −12 1 wt % Rhodorsil 2074 Example 4Compound 98 −211 None None 1 wt % Rhodorsil 2074 1 wt % USP90MD Example4 Compound 89 −222 None None 1 wt % Rhodorsil 2074 1 wt % Benzopinacole

1. A linear or cyclic siloxane compound that contains at least one Si—Hbond, at least one pendant carbon to carbon double bond, and at leastone pendant oxetane functionality.
 2. The compound according to claim 1having the structure

in which x, y, and z represent the mol % of each site to a total of100%, and q is an integer from 1 to
 50. 3. The compound according toclaim 2 having the structure

in which x represents 44 mol %, y represents 30 mol %, and z represents26 mol %.
 4. The compound according to claim 2 having the structure

in which x represents 43 mol %, y represents 42 mol %, and z represents15 mol %.
 5. The compound according to claim 1 having the structure

in which x, y, and z represent the mol % of each site to a total of100%, and q is an integer from 1 to
 50. 6. The compound according toclaim 5 having the structure

in which x represents 39 mol %, y represents 28 mol %, and z represents33 mol %.
 7. The compound according to claim 5 having the structure

in which x presents 100 mol %.